Electrodes have been used for a variety of applications, including interconnects, batteries, transparent devices (transparent electrodes), and supercapacitors. Each of these electrodes is useful in a variety of applications. For example, transparent conducting materials are useful for touch screens, solar cells, and flat panel displays.
A common material used for fabricating such electrodes is indium tin oxide (ITO). However, for low-cost or flexible electronics such as those using organic semiconductors, ITO has undesirable characteristics, such as requiring high-temperature processing steps that are incompatible with plastic substrates, brittleness, and rapidly increasing costs of scarce materials.
Attempts to meet desirable electrode characteristics, such as conductivity and transparency characteristics, for a variety of materials have been limited. For instance, certain electrodes exhibit non-uniform surface coverage on the nanometer scale, may also require lithography for fabrication, and nanowire electrodes may suffer from high roughness as the nanowires can stack hundreds of nanometers high. Additionally, surface environmental stability can be of concern with some metals and for use with organic active layers. While certain electrodes can have a reduced or absent surface dipole barrier to charge injection, many electrodes exhibit such barriers. In addition, many electrodes do not transmit much visible light when layered thickly enough to be sufficiently conducting. Other electrodes require high-temperature annealing, and can be too resistive for many applications.
Some types of electrodes use carbon nanotubes or a carbon nanotube (CNT) network. However, such networks can experience problems relating to a relatively large inter-tube junction resistance, especially between metallic and semiconducting tubes. Another such problem relates to the deposition of CNT networks, which has been challenging to implement due to inhomogeneous, inefficiently networked films. In addition, applicable methods of deposition may require the use of a large amount of surfactant that needs to be removed, can limit the size of films to the size of the membrane filter, and may involve an undesirable transfer printing step. Such electrodes can be particularly difficult to implement as transparent electrodes.
These and other issues remain as a challenge to a variety of methods, devices and systems that use or benefit from nanotube-based electrodes.